The night king struck down the dragon with ease and now the white walkers have a dragon fighting for them.
The wall isnt going to hold them back any longer and the presumably most effective weapon against them turned out to be very vulnerable.
To kill the now undead dragon isnt going to be an easy task
Killing the undead Viserion is actually quite easy, in theory. Just kill the Night King and the rest of the undead will just crumble to dust. The difficult part will be actually getting close enough to the Night King.